Tropical Instability Waves Research Articles

Air–Sea Coupling Feedbacks over Tropical Instability Waves

Abstract Tropical instability waves (TIWs) are oceanic features that form around the equatorial Pacific cold tongue and influence the large-scale circulation and coupled climate variability including El Niño–Southern Oscillation. Local air–sea coupling over TIWs is thought to play an important role in the atmosphere and ocean’s energy and tracer budgets but is not well captured in coarse-resolution models. In this study, we isolate the impacts of TIW thermal (sea surface temperature–driven) and current (surface current–driven) feedbacks by removing TIW signatures in air–sea coupling fields in a high-resolution regional coupled model. The thermal feedback is found to damp TIW temperature variance by a factor of 2, associated both with the direct dependence of surface heat fluxes on SST (∼74%) and indirect impacts on surface winds (∼35%) and air temperature and humidity (∼−9%). These changes lead to cooling of the cold tongue SST by up to 0.1°C through reduced TIW-driven meridional heat fluxes and associated small changes in atmospheric circulation. The current feedback is decomposed into TIW (i.e., mesoscale) and mean (i.e., large-scale) components using separate experiments, with both having distinct impacts on TIWs and the mean state. The mesoscale current feedback reduces TIW eddy kinetic energy (EKE) by 22% through the eddy wind work, while the mean current feedback induces a further reduction of 8% by extracting energy from the mean currents and thus reducing barotropic EKE shear production. An improved understanding of small-scale tropical Pacific processes is needed to address biases in coarse-resolution models that impact their predictions and projections of Pacific climate variability and change. Significance Statement Tropical instability waves (TIWs) are oceanic features with ∼1000-km wavelengths that propagate westward on either side of the eastern equatorial Pacific cold tongue. TIWs drive lateral and vertical heat fluxes that impact several aspects of El Niño–Southern Oscillation. While climate models with a moderate, 1/4° ocean resolution can capture some TIW variability, they fail to properly represent many associated processes such as the impact of TIWs on the overlying atmosphere. Using sensitivity studies performed using a high-resolution regional coupled model, we study the impact of TIW air–sea coupling on the eastern Pacific climate system. Increased understanding of small-scale processes from studies such as this is essential to understand and address biases in models used for seasonal climate predictions and projections in the Pacific region.

Seasonality of Subsurface Shear Instabilities at Tropical Instability Wave Fronts in the Atlantic Ocean in a High‐Resolution Simulation

AbstractTropical Instability Waves (TIWs) have been shown to modulate upper ocean mixing. However, previous studies on the modulation of TIW related mixing are based on small numbers of TIWs and have not considered temporal variability, which can lead to discrepancies in the findings. In this study, using a 12‐year simulation carried out with a comprehensive, global, high‐resolution ocean model, we present evidence of seasonally modulated shear instabilities at TIW fronts in the Atlantic Ocean that reach down to the thermocline depth, potentially inducing mixing below the mixed layer depth. We find that, regardless of whether TIWs are present earlier in the year, frontal instabilities and potential mixing primarily occur in boreal summer, coinciding with a vertical shear maximum between the mean zonal currents. We argue that in the Atlantic Ocean, vertical shear at TIW fronts does usually not suffice to cause frontal instabilities below the mixed layer depth. Instead, the background shear needs to be sufficiently large in addition to TIW shear, to overcome the stability. The background shear in turn varies seasonally and is strongly driven by the variability of the northern branch of the South Equatorial Current (nSEC). As such, the variability of the nSEC strongly contributes to the generation and modulation of instabilities at TIW fronts that reach below the mixed layer depth and have the potential to induce mixing. Our results highlight the importance of seasonal variability when studying TIW impacts and their effect on mixing.

Why Do Oceanic Nonlinearities Contribute Only Weakly to Extreme El Niño Events?

AbstractExtreme El Niño events have outsized global impacts and control the El Niño Southern Oscillation (ENSO) warm/cold phases asymmetries. Yet, a consensus regarding the relative contributions of atmospheric and oceanic nonlinearities to their genesis remains elusive. Here, we isolate the contribution of oceanic nonlinearities by conducting paired experiments forced with opposite wind stress anomalies in an oceanic general circulation model, which realistically simulates extreme El Niño events and oceanic nonlinearities thought to contribute to ENSO skewness (Tropical Instability Waves (TIWs), Nonlinear Dynamical Heating (NDH)). Our findings indicate a weak contribution of oceanic nonlinearities to extreme El Niño events in the eastern Pacific, owing to compensatory effects between lateral (NDH and TIWs) and vertical processes. These results hold across different vertical mixing schemes and modifications of the upper‐ocean heat budget mixed layer criterion. Our study reinforces previous research underscoring the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events.

Eddy‐Mediated Turbulent Mixing of Oxygen in the Equatorial Pacific

AbstractIn the tropical Pacific, weak ventilation and intense microbial respiration at depth give rise to a low dissolved oxygen (O2) environment that is thought to be ventilated primarily by the equatorial current system (ECS). The role of mesoscale eddies and vertical mixing as potential pathways of O2 supply in this region, however, remains poorly known due to sparse observations and coarse model resolution. Using an eddy resolving simulation of ocean circulation and biogeochemistry, we assess the contribution of these processes to the O2 budget balance and find that vertical mixing of O2, which is modulated by the surface wind speed and the vertical shear of the eddying currents, contributes substantially to the replenishment of O2 in the upper equatorial Pacific thermocline, complementing the advective supply of O2 by the ECS and meridional circulation at depth. These transport processes vary seasonally in conjunction with the wind: mixing of O2 into the upper thermocline is strongest during boreal summer and fall when the vertical shear and eddy kinetic energy are intensified. The relationship between eddy activity and the downward mixing of O2 arises from the modulation of equatorial turbulence by Tropical Instability Waves via their impacts on the vertical shear. This interaction of processes across scales sustains a local pathway of O2 delivery into the equatorial Pacific interior and highlights the need for adequate observations and models of turbulent mixing and mesoscale processes for understanding and predicting the fate of the tropical Pacific O2 content in a warmer and more stratified ocean.

Pacific tropical instability waves have intensified since the 1990s

Pacific tropical instability waves have intensified since the 1990s

Intensification of Pacific tropical instability waves over the recent three decades

Intensification of Pacific tropical instability waves over the recent three decades

Time‐Dependent Role of Multiyear La Niña in Impacting the Pacific Tropical Instability Waves

AbstractPrevious studies have shown that the tropical instability waves (TIWs) in the Equatorial Pacific are enhanced during La Niña and suppressed during El Niño. Unlike El Niño which tends to decay quickly, La Niña often persists through the subsequent one or 2 years, becoming a multiyear La Niña. Using a time‐dependent and space‐localized energetics formalism and a suite of observational and reanalysis data, we found that the eddy kinetic energy (EKE) associated with TIWs only peaks during the first‐year La Niña and decreases significantly during the second‐year La Niña. This is caused, to the first order, by the weaker equatorial cold tongue near the equator and anomalous cooling in the off‐equator regions during the second‐year La Niña, as compared to the first‐year La Niña, which leads to a significant decrease in meridional density shear, and thus a reduction in baroclinic instability. Meanwhile, the weakened South Equatorial Current during the second‐year La Niña also contribute to EKE decrease by transferring less kinetic energy to the TIWs via barotropic instability. The meridionally broad spatial pattern of negative temperature anomalies during the second‐year La Niña is the major reason for the weakened TIW activity.

Modulation of Equatorial Currents and Tropical Instability Waves During the 2021 Atlantic Niño

AbstractIn the boreal summer of 2021, the equatorial Atlantic experienced the strongest warm event, that is, Atlantic Niño, since the beginning of satellite observations in the 1970s. Such events have far‐reaching impacts on large‐scale wind patterns and rainfall over the surrounding continents. Yet, developing a paradigm of how Atlantic Niño interacts with the upper‐ocean currents and intraseasonal waves remains elusive. Here we show that the equatorial Kelvin wave associated with the onset of the 2021 Atlantic Niño modulated both the background flow and the eddy flux of the equatorial upper‐ocean circulation, causing an extremely weak and delayed tropical instability wave (TIW) season. TIW‐induced variations of sea surface temperature (SST), sea surface salinity, sea surface height, and eddy temperature advection were exceptionally weak during May to July, the climatological peak of TIW activity, but rebounded in August when higher than normal variability was observed. Moored velocity data at 23°W show that during the peak of the 2021 Atlantic Niño from June to August, the Equatorial Undercurrent was deeper and stronger than usual. An anomalously weak eddy momentum flux strongly suppressed barotropic energy conversion north of the equator from May to July, likely contributing to low TIW activity. Reduced baroclinic energy conversion also might have played a role, as the meridional gradient of SST was sharply reduced during the Atlantic Niño. Despite extremely weak TIW velocities, modest intraseasonal variability of chlorophyll‐a (Chl‐a) was observed during the Atlantic Niño, due to pronounced meridional Chl‐a gradients that partly compensated for the weak TIWs.

On the Contribution of Transient Diabatic Processes to Ocean Heat Transport and Temperature Variability

Abstract Time-varying processes contribute to ocean heat transport and are important to understand for accurate climate modeling. While past studies have quantified time-varying contributions to advective transport, less attention has been given to diabatic processes such as surface forcing and mixing. Using a global eddy-permitting ocean model we quantify the contribution of time-variable processes to meridional and diathermal (warm to cold) heat transport at different time scales using a temporal eddy-mean decomposition performed in the temperature–latitude plane. Time-varying contributions to meridional heat transport occur predominantly at mesoscale eddy-dominated midlatitudes and in the tropics, associated with the seasonal cycle and tropical instability waves. The seasonal cycle is a dominant driver of surface flux– and mixing-driven diathermal heat transports. Nonseasonal (and nondiurnal) processes contribute up to about 10% of the total. We show that transient contributions to diathermal heat transport can be interpreted as sources of Eulerian temperature variance. We thus extend recent work on the drivers of temperature variability by evaluating the role of mixing. Mixing dampens seasonal and diurnal temperature variability, except near the equator where it can be a source of seasonal variability. At mesoscale time scales mixing drives variability within and near the base of the boundary layer, the mechanisms of which are explored using a column model. We suggest that climate models that do not resolve the mesoscale may be missing the rectified heat transport associated with high-frequency diabatic processes, in addition to the adiabatic eddy fluxes that are commonly parameterized. Significance Statement Ocean heat transport plays a key role in determining how the climate responds to changes in forcing. This transport is influenced by a range of processes that vary with time. Previous research has quantified time-varying sources of lateral heat transport, such as mesoscale eddies and overturning circulation cells. However, time-varying “diabatic processes,” such as surface forcing and unresolved turbulent mixing, have received less attention. Here, we quantify these effects using a global ocean model. We find a dominant role for the seasonal cycle in driving diabatic heat transport, but processes on shorter time scales also contribute. Our results suggest that temporal variations in turbulent mixing are an important contributor to heat transport but may not be resolved in coarse-resolution climate models.

Ocean mixing induced by three-dimensional structure of equatorial mode tropical instability waves in the Pacific Ocean

Observations have revealed that tropical instability waves (TIWs) play an important role in the ocean mixing of the equatorial thermocline. However, most studies have not distinguished the individual effects from the TIWs’ two modes, and most of them focused on the effect of the TIW-induced vertical shear on the formation of mixing. In this study, we utilize the high-resolution ocean reanalysis data to isolate the 3D structure of the equatorial mode of TIW (eTIW) and examine the detailed mechanisms of its mixing effects. Horizontally, the temperature associated with the eTIWs shows a centrosymmetric structure in the upper layers, but vertically, the phase begins to transition at ~70 and becomes opposite to that of the upper layer below 90 m. The centrosymmetric temperature structure leads to a similar stratification pattern in the upper layers. We find that the pattern of the eTIW-associated reduced shear squared (RSS), an indicator for potential shear instability, is controlled by its stratification rather than shear, suggesting that the former plays a more important role than the latter in resulting in shear instability and mixing. Specifically, it is in the northern (southern) part of the clockwise (anticlockwise) phase of the eTIWs that the eTIW-associated RSS is positively large, where the total flow is more likely to be modulated by shear instability and mixing. Hence, the present study provides a new mechanism for the TIW-induced mixing in the equatorial thermocline. However, in specific regions where the eTIW-induced shear has the same sign as that of the mean flow, the eTIW-associated shear squared can also significantly enhance the total RSS and lead to shear instability, consistent with previous studies.

Contrasts of bimodal tropical instability waves (TIWs)-induced wind stress perturbations in the Pacific Ocean among observations, ocean models, and coupled climate models

Contrasts of bimodal tropical instability waves (TIWs)-induced wind stress perturbations in the Pacific Ocean among observations, ocean models, and coupled climate models

Internal Solitary Waves within the Cold Tongue of the Equatorial Pacific Generated by Buoyant Gravity Currents

Abstract The equatorial cold tongue in the Pacific Ocean has been intensely studied during the last decades as it plays an important role in air–sea interactions and climate issues. Recently, Warner et al. revealed gravity currents apparently originating in tropical instability waves. Both phenomena have strong dissipation rates and were considered to play a significant role in cascading energy from the mesoscale to smaller horizontal scales, as well as to vertical scales less than 1 m. Here, we present Sentinel-3 satellite observations of internal solitary waves (ISWs) in the Pacific cold tongue near the equator, in a zonal band stretching from 210° to 265°E, away from any steep bottom topography. Within this band these waves propagate in multiple directions. Some of the waves’ characteristics, such as the distance between wave crests, crest lengths, and time scales, are estimated from satellite observations. In total we identify 116 ISW trains during one full year (2020), with typical distances between crests of 1500 m and crest lengths of hundreds of kilometers. These ISW trains appear to be generated by buoyant gravity currents having sharp fronts detectable in thermal infrared satellite images. A 2D numerical model confirms that resonantly generated nonlinear internal waves with amplitudes of O(10) m may be continuously initiated at the fronts of advancing gravity currents. Significance Statement Satellite imagery reveals the repeated occurrence of internal solitary waves in the near-equatorial region of the east Pacific, despite the absence of topography. These waves appear to be resonantly generated over the sheared Equatorial Undercurrent by gravity currents that propagate as frontal zones of 1000-km scale tropical instability waves, providing a physical link with viscous mixing scales.

Effective generation mechanisms of tropical instability waves as represented by high-resolution coupled atmosphere–ocean prediction experiments

Cusp-shaped fluctuations of the sea surface temperature (SST) front in the tropical Pacific, now known as tropical instability waves (TIWs), were discovered by remote sensing in the 1970s. Their discovery was followed by both theoretical and analytical studies, which, along with in situ observations, identified several possible generation mechanisms. Although modeling studies have shown that TIWs strongly influence the heat budget, their influence on local variations of realistically initialized predictions is not yet understood. We here evaluate a series of medium-range (up to ~ 10 days) coupled atmosphere–ocean predictions by a coupled model with different horizontal resolutions. Observational SST, surface wind stress, heat flux, and pressure data showed that representation of temporally and spatially local variations was improved by resolving fine-scale SST variations around the initialized coarse-scale SST front fluctuations of TIWs. Our study thus demonstrates the advantage of using high-resolution coupled models for medium-range predictions. In addition, analysis of TIW energetics showed two dominant sources of energy to anticyclonic eddies: barotropic instability between equatorial zonal currents and baroclinic instability due to intense density fronts. In turn, the eddy circulation strengthened both instabilities in the resolved simulations. This revealed feedback process refines our understanding of the generation mechanisms of TIWs.

The Nature of Opal Burial in the Equatorial Atlantic During the Deglaciation

AbstractRelatively high opal concentrations are measured in equatorial Atlantic sediments from the most recent deglaciation. To shed light on their causes, seven cores were analyzed for their content of siliceous (diatom, silicoflagellates, radiolarians, phytoliths, and sponge spicules) and calcareous (coccolithophores) microfossils. An early deglacial signal is detected at the time of rising boreal summer insolation ca. 18 ka by the coccolithophores. The surface freshening is likely due to the rain belt associated with the intertropical convergence zone (ITCZ), implying its southward shift relatively to its present‐day average positioning. The diatom assemblages corresponding to the following increase in diatom abundances ca. 15.5 ka suggest the formation of a cold tongue of upwelled water associated with tropical instability waves propagating westward. Such conditions occur at present during boreal summer, when southerly trade winds are intensified, and the ITCZ shifts northward. The presence of the diatom Ethmodiscus rex (Wallich) Hendey and the coccolithophore Florisphera profunda indicates a deep thermocline and nutrient enrichment of the lower photic zone, revealing that Si‐rich southern sourced water (SSW) likely contributed to enhanced primary productivity during this time interval. The discrepancies between the maximum opal concentrations and siliceous marine microfossils records evidence the contribution of freshwater diatoms and phytoliths, indicative of other processes. The definition of the nature of the opal record suggests successive productivity conditions associated with specific atmospheric settings determining the latitudinal ITCZ positioning and the development of oceanic processes; and major oceanic circulation changes permitting the contribution of SSW to marine productivity at this latitude.

Mechanisms for generating and connecting the Yanai-mode and Rossby-mode tropical instability waves in the equatorial Pacific

It is generally believed that both barotropic and baroclinic instabilities may account for the tropical instability wave (TIW) variabilities in the equatorial Pacific Ocean. However, the coupling mechanisms between the two different meridional TIW modes, i.e., the Yanai-mode at the equator and the Rossby-mode around 5°N, and the associated vertical connections, are yet to be clarified. This is done here using a recently developed multiscale decomposition tool, multiscale window transform (MWT), and the MWT-based theory of canonical transfer and TIW-scale eddy kinetic energy (EKE) budget analysis, based on the (1/12.5)∘ HYCOM reanalysis data. In the subsurface layer (30–200 m), barotropic instability is the primary energy source for the Yanai-mode TIWs, while baroclinic instability dominates the EKE generation of the Rossby-mode TIWs. In contrast, the two TIW modes in the surface layer (0–30 m) are largely modulated by nonlocal mechanisms through pressure work, which functions to transport EKE upward from the subsurface to the surface layer, and southward from the Rossby-mode region to the Yanai-mode region. The pressure fluxes substantially couple the two TIW modes in both the meridional and vertical directions, leading to significant coherence between EKE in different vertical layers and regions of the TIWs. In addition, the prevailing vertical pressure flux identified north of the equator explains why the EKE signal is more vertically coherent in the Rossby-mode region than the Yanai-mode region. This nonlocal energy pathway, as well as the well-known instability processes, undergoes strong seasonal variations that determine the seasonal cycles of the TIWs in the equatorial Pacific Ocean.

Parameterizing the nonlinear feedback on ENSO from tropical instability waves (TIWs) by nonlinear eddy thermal diffusivity

Parameterizing the nonlinear feedback on ENSO from tropical instability waves (TIWs) by nonlinear eddy thermal diffusivity

Tropical instability waves in a high-resolution oceanic and coupled GCM

Tropical instability waves (TIWs) are the dominant mesoscale variability in the eastern equatorial Pacific Ocean. TIWs have direct impacts on the local hydrology, biochemistry and atmospheric boundary layer, and feedback on ocean circulations and climate variability. In this study, the basic characteristics of Pacific Ocean TIWs simulated by an eddy-resolving ocean model and a coupled general circulation model are evaluated. The simulated TIW biases mainly result from the mean climatology state, as TIWs extract eddy energy from the mean potential and kinetic energy. Both the oceanic and coupled models reproduce the observed westward propagating large-scale Rossby waves between approximately 2°–8°N, but the simulated TIWs have shorter wavelengths than the observed waves due to the shallower thermocline. Meanwhile, the weak meridional shears of background zonal currents and the less-tilted pycnocline in these two models compared to the observations causes weak barotropic and baroclinic instability, which decreases the intensity of the simulated TIWs. We then contrast the TIWs from these two models and identify the roles of atmospheric feedback in modulating TIWs. The latent heat flux feedback is similar to observation in the coupled model but absent in the ocean model, contributing to the stronger standard deviation (STD) of the TIW SST in the ocean model. The ocean model is not able to capture realistic air–sea interaction processes when forced with prescribed atmospheric forcing. However, the misrepresented atmospheric feedback in the ocean model tends to decrease the sea surface height (SSH) variability, and the current feedback damping effect is stronger in the ocean model than in the coupled model. Combined with weaker barotropic conversion rate and baroclinic conversion rate in the ocean model than in the coupled model, the STD of the TIW SSH in the ocean model is weaker.

Synchronous Chlorophyll-a and Sea Surface Salinity Variability in the Equatorial Pacific Ocean

Using chlorophyll-a (Chl-a) concentration data derived from the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (SNPP), the in situ measurements from the Tropical Ocean Atmosphere (TAO) moorings, and the sea surface salinity (SSS) data from the Soil Moisture Active Passive (SMAP) mission, we report synchronous Chl-a and SSS variability in the Equatorial Pacific Ocean on the monthly and daily bases. During the El Niño event in 2015, a decrease in Chl-a and SSS occurred and developed within the same timeframe, and possessed similar spatial patterns across the Equatorial Pacific Ocean. Enhanced Chl-a and SSS coincided and co-located (in timing, location, spatial coverage, and extent) during the La Niña event in 2020. In contrast, sea surface temperature (SST) variability did not relate to Chl-a and SSS variability across the Equatorial Pacific Ocean. Chl-a and SSS were found to covary on the daily basis driven by the tropical instability waves (TIWs). The mechanism that caused the synchronous Chl-a and SSS variability in the Equatorial Pacific Ocean on both the daily and monthly bases is addressed and discussed.

Simulation Framework and Case Studies for the Design of Sea Surface Salinity Remote Sensing Missions

<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> -band microwave radiometers have now been used to measure sea surface salinity (SSS) from space for over a decade with the SMOS, Aquarius, and SMAP missions, and it is expected that the launch of the CIMR mission in the later half of this decade will ensure measurement continuity in the near future. Beyond these missions, it is useful to consider how future missions can be designed to meet different scientific objectives and performance requirements as well as to fit within different cost spaces. In this article, we present a software simulator for remote sensing measurements of ocean state capable of generating L1- and L2- equivalent data products for an arbitrary spacecraft mission including multifrequency fixed-pointing or scanning microwave radiometers.This simulator is then applied to case studies of SSS measurement over selected areas of interest, including the Gulf Stream, Southern Ocean, and Pacific tropical instability wave regions. These simulations illustrate how different design choices concerning receiver bandwidth and revisit time can improve the detection of SSS features in these regions from the mesoscale to the seasonal scale.

Modulation of the Eastern Equatorial Pacific Seasonal Cycle by Tropical Instability Waves

AbstractFeedbacks from tropical instability waves (TIWs) on the seasonal cycle of the eastern Pacific Ocean are studied using two eddy‐rich ocean simulations, with and without TIWs. By warming the equatorial waters by up to 0.4°C through nonlinear advection in boreal summer and fall, TIWs reduce the amplitude of the seasonal cycle in upper ocean temperatures. In addition, TIWs stabilize the upper part of the Equatorial Undercurrent (EUC) through enhanced barotropic energy conversion, leading to a year‐round weakening by −0.15 m s−1 and preventing an unrealistic re‐intensification in boreal fall usually found in non‐eddy resolving models. A coarser simulation at 1‐degree horizontal resolution fails to reproduce the TIW‐induced nonlinear warming of equatorial waters, but succeeds in inhibiting the EUC re‐intensification. This suggests a threshold effect in TIW strength, associated with the model's ability to simulate eddies, which may be responsible for long‐standing biases displayed by global climate models in this region.